1. Field of the Invention
The present invention relates to a laser processing method for processing a workpiece such as a semiconductor wafer.
2. Description of the Related Art
In a semiconductor device fabrication process, a plurality of first division lines and a plurality of second division lines intersecting the first division lines are formed on the front side of a platelike workpiece such as a semiconductor wafer to thereby partition a plurality of regions where a plurality of functional devices are respectively formed. The workpiece thus formed with the functional devices is divided along the first division lines and the second division lines to thereby produce a plurality of individual chips.
As a method of dividing the workpiece along the first division lines and the second division lines, a laser processing method is known, wherein a pulse-shaped processing laser beam (pulsed laser beam) in an infrared region having a transmission wavelength to the workpiece is used and this pulsed laser beam is applied to the workpiece so as to be focused inside the workpiece (see Japanese Patent No. 3408805 and Japanese Patent Laid-open No. 2005-332841, for example). In this dividing method using laser processing, the pulsed laser beam is applied by laser applying means from one side of the workpiece so as to be focused inside the workpiece, thereby continuously forming a modified region inside the workpiece along each division line. Thereafter, an external force is applied to the workpiece to thereby break the workpiece along each division line where the strength of the workpiece has been reduced by the formation of the modified region, thus dividing the workpiece into a plurality of individual chips.
Further, Japanese Patent Laid-open No. 2006-171530 discloses a technique such that a plurality of first modified regions (modified layers) are formed along the first and second division lines in a workpiece at its central portion in the direction of the thickness of the workpiece and that a plurality of second modified regions thicker than the first modified regions are formed at a plurality of intersecting regions between the first division lines and the second division lines, thereby preventing a reduction in die strength of each chip divided from the workpiece.